The current invention is concerned with magnetic data stripes, and in particular optically variable magnetic stripe assemblies, such as those found on financial transaction cards.
It has been conventional practice now for many years, to provide a magnetic stripe on payment and identity documents such as credit cards, debit cards, cheque cards, transport tickets, savings books and other forms of security documents. The presence of the magnetic stripe allows such documents to become carriers of non-visual machine readable data.
In many instances such documents have also been provided with a visual security or authentication device in the form of an embossed hologram or diffractive image. However the presence of both such devices on such documents significantly reduces the remaining surface area of document available to carry other information, security features and design elements.
There has therefore been a drive to combine the two devices in one integrated structure, which we refer to henceforth as an optically variable-magnetic (OVM) stripe. The resultant device may be regarded as either a visually secured magnetic data carrier or alternatively a Ahologram@ which can be personalised with machine readable data (and read in an open architecture environment).
Prior art constructions for OVM stripes have been detailed U.S. Pat. No. 4,684,795, U.S. Pat. No. 4,631,222, U.S. Pat. No. 5,383,687 and EP-A-0998396. In principle the OVM stripe can substitute for all applications where currently high and low coercivity tape is currently applied, the most significant application by value is that in which the OVM stripe is applied to plastic financial transaction cards.
FIG. 1 is a cross-sectional schematic of a conventional prior art OVM stripe applied to a financial card as described in the prior art cited above. Essentially it comprises 2 functional sub-structures:
1. A transparent lacquer layer 1 embossed with an holographic or diffractive surface relief structure 2 and coated with a continuous reflection-enhancing layer of metal 3, typically aluminium, bonded by an adhesion promoting primer layer 4 to
2. A magnetic layer 5 that is coated on the primer layer 4. The magnetic layer 5 is further coated with a heat activated adhesive layer 6 to bond the structure to the card substrate 7.
The plastic transaction card 7 is typically a tri-laminate structure (not shown) comprising an opaque central polymeric core layer printed with information on either side, laminated between 2 transparent polymeric overlay sheets.
The OVM stripe is first applied to that transparent overlay sheet pertaining to the rear of the card, by a heat activated continuous roll-on transfer process. Subsequent to this the three laminate layers are then fuse bonded together in a laminating press. In order to apply the magnetic tape to the transparent overlay sheet, through in essence a hot-stamping process, it is first necessary to provide the OVM stripe structure onto a release coated carrier or backing layer.
However a structural drawback of the prior art OVM stripe has been identified. Unlike conventional non-holographic magnetic stripes the prior art OVM stripes are provided with a continuous metallic reflection enhancing layer 3. This metallic reflection enhancing layer is conductive and this has led to problems with static discharges in automatic teller machines.
It is well known that under conditions of low environmental humidity, substantial electrostatic surface charges can build up on articles or bodies which are poor conductors or conversely good insulators. For example a person walking around in a carpeted room wearing shoes with insulating (e.g. rubber) soles, can acquire a very significant amount of electrostatic surface charge this will become evident when that person touches a good conductor such a metal door handle, thus effecting rapid discharge of this electrostatic charge and experienced as a minor electric shock.
In particular as the air humidity drops below 25% the conductivity of the air becomes low enough to prevent any leakage of electrostatic charge into the atmosphere in such circumstances electrostatic potentials in excess of several kilovolts can build up on the human body.
Consider next a plastic, typically PVC, transaction card containing a conventional magnetic stripe.
PVC when compared to the human body is a very good insulator, hence we should expect, in absence of a conductive element within the card making contact with a second conductor external to the card, that there will be a distribution of electrostatic charge on the surface of the card.
Now the magnetic oxide layer within the known non-holographic magnetic stripe is currently exposed at either edge of the card and hence there exists the potential that when the card is inserted into an automated transaction machine (ATM) or magnetic card reader the exposed edge may contact a conductive component within the reader and rapidly discharge the electrostatic build-up on the surface of the card into the electrical circuitry of the ATM or reader. The associated voltage spike may be sufficiently large to damage or de-activate the machine. However tests conducted by the inventors have confirmed that the conductivity of the magnetic oxide layer is poor resulting at worst in a very slow transfer or discharge of the electrostatic potential built up on the card.
However moving our consideration of this electrostatic discharge problem on from the scenario of using a card containing a standard magnetic stripe to that where the card contains an OVM stripe, we now have the opportunity for conduction and thus electrostatic discharge through the reflective metal layer 3 applied to the surface relief 2 present on the holographic diffractive layer. Tests conducted by the inventors, wherein the exposed edge of the OVM stripe (present on a PVC transaction card) is brought into contact with a metal sphere connected to a device capable of measuring the transit dynamic changes in the charge or voltage transferred to the metal sphere confirm that the reflective metal layer very rapidly and efficiently discharges the electrostatic charge that had resided on the exterior of the card onto the metal sphere.
Furthermore such tests also confirm that if an individual holds the card in such a way that one finger contacts the near edge of the OVM stripe, whilst the other end of the OVM stripe is allowed to touch the conducting sphere then whatever electrostatic charge and potential is present on the individual will also be rapidly discharged onto the conducting sphere.
Clearly since the electrostatic build up on an individual under the right environmental conditions can be very considerable, there is therefore a significant risk that when a transaction card is located into an ATM or reader in the manner described (causing discharge of the electrostatic present on card and card holder into the circuitry of the machine) the machine may be damaged or its operation disrupted.
In accordance with a first aspect of the present invention, an optically variable magnetic stripe assembly includes a magnetic layer, an optically variable effect generating layer over the magnetic layer, and an electrically non-conductive reflective layer between the magnetic layer and the optically variable effect generating layer.
The inventors recognised that a modified OVM stripe structure was required in order to eliminate risk in the field that cards containing an OVM stripe may cause operational problems associated with electrostatic discharge through the metal layer and in particular end discharge electrically linking the body of the card holder to conductive elements in the transaction device or reader.
In the first aspect of the invention, we replace the metal reflecting layer of the prior art with an electrically non-conductive reflective layer. This then reduces or avoids the problem of electrical discharge when the edge of a security document provided with the magnetic stripe assembly is touched.
The non-conductive reflective layer can be fabricated in a number of ways by, for example, using a non-metallic material such as a high refractive index material.
In accordance with a second aspect of the present invention, an optically variable magnetic stripe assembly includes a magnetic layer, an optically variable effect generating layer over the magnetic layer, and a reflective layer between the magnetic layer and the optically variable effect generating layer, the reflective layer comprising at least one metal portion, the or each metal portion only partially extending along the full length of the optically variable effect generating layer.
In this aspect, a metal reflective layer can be used but in the form of at least one metal portion and by ensuring there is no electrically conductive path along the full length of the optically variable effect generating layer. This could be achieved by providing a number of metal portions with discrete breaks between them or by ensuring that the metal portion does not extend to the edges of the assembly.
In accordance with a third aspect of the present invention, an optically variable magnetic stripe assembly includes a magnetic layer, an optically variable effect generating layer over the magnetic layer, a discontinuous metal reflective layer between the metal layer and the optically variable effect generating layer, and a static resistive layer positioned to enable a static charge to be dissipated in a controlled manner.
The static resistive layer may contact the metal reflective layer but this is not essential. The metal layer can be discontinuous by incorporating discrete breaks between metal portions or by using a dot demet structure.
In this approach, a static resistive layer (or high resistance) layer allows charge slowly to be dissipated in a controlled manner. Such a “static resistive layer” needs to have a surface resistivity in the range 10e6-10e10 ohms/square? but especially 10e8-10e9 ohms/square. Suitable static resistive layers comprise a combination of a electroconductive pigment in a non-conducting binder. Examples of suitable conductive pigments include Carbon black and Antimony Oxide. VMCH is an example of a suitable binder. (VMCH is a commonly used binder/adhesive available from a number of sources e.g. http://www.dow.com/svr/prod/cmvc.htm).
The magnetic stripe assembly can be used with a variety of security articles including security documents as will be readily apparent to a person of ordinary skill in the art.
In the following description, those layers which are substantially the same as layers described earlier will be given the same reference numerals.